The minerals in the feldspar family make up more than half of the Earth's rocky crust. Occasionally these common minerals form crystals that shimmer like the light of the moon or a rainbow on a soap bubble. Called iridescence, this phenomenon is caused by light scattering, or diffracting, off closely spaced layers of different composition in the feldspar crystals. The gems cut from these iridescent crystals are called moonstone, sunstone, and labradorite. This 35.38 carat moonstone is from Tanzania. Moonstones are prized for their beautiful blue iridescence, known as adularescence. Flawless, clear, or translucent gems exhibiting a rich blue sheen are most valuable. Moonstones are typically cut as cabochons to best show off the effect. The finest moonstone gems come from Sri Lanka, Burma, and India. This is the first moonstone from Tanzania for the National Gem Collection.